You are sent back to the Middle Ages in a time machine

You are sent back to the Middle Ages in a time machine the size of a telephone booth. You can bring exactly one modern object back with you to help you survive and thrive during that time. Besides that object, nothing else but the clothes you're currently wearing can come with you. The object you choose can contain only the parts and accessories that are normally part of that object, such as wires or batteries, but nothing else. What do you bring? WHY is this significant for your survival and HOW will this object be beneficial within society during that era?